Propane is a liquified petroleum gas and aromatic hydrocarbon that may be utilized as a gaseous fuel. Unlike methane vapor that is lighter than air, propane vapor is heavier than air. Liquid propane will vaporize at any temperature above −44° F. Propane, and all other hydrocarbon-based fuels, must be kept away from open flames and ignition sources. Propane must be handled with care and safety, particularly when filling propane tanks. Propane is sold as a liquid by the pound or as a vapor by the cubic foot. As ambient temperature rises, propane vapor pressure rises. When ambient temperature drops, propane vapor pressure drops.
Propane is typically sold and stored in portable containers for home use. When a container is filled to its proper liquid level, it will be approximately 80% full. The remaining 20% of the container's volume contains propane vapor. Propane vapor is typically consumed by household appliances. The vapor space in the container also provides room for the expansion of liquid propane. Liquid propane has a moderate coefficient of expansion and, accordingly, expands greatly when subjected to temperature increases. If a container is filled beyond its maximum permitted liquid filling density, a situation may develop in which there will not be enough space available to accommodate the liquid propane's expansion. When an overfilled container is exposed to any temperature increase, hazardous conditions may occur. Consequently, a need exists for a propane tank filling system which is able to fill a propane tank to its appropriate level economically and safely.